1. Field of the Invention
The present invention is related to a papermaking machine of the type wherein a plurality of dewatering elements remove water from stock which has been deposited on a forming medium from a slice of a headbox. The machine includes a non-dewatering section in the form of a forming board positioned between the slice and the dewatering elements and a shower apparatus for showering the stock with a plurality of needle jets of water. The needle jets extend in a downstream direction, form a small acute angle with the stock, and are in close proximity with the stock so that the needle jets of water gently impinge upon the stock as continuous streams at a position where the stock is carried over the forming board by the forming medium.
2. Description of the Prior Art
In the typical Fourdrinier papermaking machine, an aqueous suspension of fibers, called the "stock" is flowed from a headbox slice onto a traveling Fourdrinier wire or medium, generally a woven belt of wire and/or synthetic material, to form a continuous sheet of paper or paper-like material. In this connection, the expression "paper or paper-like material" is used in a broad or generic sense and is intended to include such items as paper, kraft, board, pulp sheets and non-woven sheet-like structures. As the stock travels along on the Fourdrinier wire, formation of a paper web occurs, as much of the water content of the stock is removed by draining. Water removal is enhanced by the use of such well-known devices as hydrofoils, table rolls and/or suction devices.
One of the problems incurred in the production of paper is that the physical characteristics of the paper web being formed generally reflect both the non-uniformities of the flow such as streaks and pulsations, and the degree of reflocculation of the stock after it has been deposited from the headbox slice onto the forming medium. Generally, the more non-uniform the flow and the greater the degree of reflocculation, the greater the tendency for the paper web being formed to have various known undesirable features. In known prior art apparatus, the stock is deposited from the slice of the headbox onto the surface of a forming medium which is travelling in the machine direction which is the direction from the headbox to the couch roll. The portion of the forming medium where the stock is so deposited is travelling over a forming board. It is known that the rate of sheet formation and of reflocculation at this position are excessively high. It is desirable to prevent sheet formation from the flow containing these flow non-uniformities and redisperse the reflocculated stock at this position prior to the initiation of sheet formation.
Efforts have been made to reduce web imperfections resulting from such flow non-uniformities and reflocculation of headbox discharge, a problem which has been especially bothersome since the advent of high speed papermaking machines which may operate above about 2000 feet per minute. In early slower Fourdrinier machines, stock was deposited onto a flexible rubber apron which prevented dewatering and sheet formation for a short distance down the forming section of the papermaking machine measured in the machine direction. In addition to an apron, early slow speed Fourdrinier machines had a shake which provided cross machine direction shear throughout the forming process, the effect of which was to disperse the stock. As machine speeds increased, the effectiveness of the shake declined. Recently, attempts have been made at introducing effective shear in high speed applications by means of what is known in the art as the serrated slice. However, problems have been incurred due to the flow non-uniformities of the headbox discharge in that the serrated slice greatly amplifies such non-uniformities. Depending upon the magnitude of the flow disturbances and the machine speed, the stock may jump, spout, roostertail or fly in the form of a spray down the papermaking machine table, undesirable occurences which are well understood by those skilled in the art.
Efforts have also been made to control the characteristics of the paper web by allowing the stock deposited upon the forming medium to "run" for a distance in the absence of dewatering and formation and to subsequently subject the stock to showering. In this manner, undesirable variations in the headbox discharge are first allowed to dissipate, and then an effort is made to redisperse the stock by means of showering. It is believed that a primary objective of such showering should be to provide a uniform flow pattern in the dissipated stock and that such pattern should be in the form of continuous closely spaced ridges which can be collapsed and regenerated in a known manner to overcome reflocculation. However, in known prior art paper-making applications the result of such showering has been to cause an undesirable degree of stock jump.
It is further believed that heretofore the creation of such stock jump has generally resulted from the type of shower used and the more or less haphazard positioning of the shower. For example, typical showers in use today in papermaking facilities are almost invariably a round pipe which delivers water from closely spaced holes drilled in the pipe rather than from nozzles. Generally, the pipes are located at a considerable height above the stock usually of the order of one to two feet. The effect is to cause the stock to be impinged by individual drops rather than continuous needle-like jets. The pipes are usually located wherever it is convenient to do so. For example, it is not at all uncommon to find shower pipes hanging from a catwalk or some other pre-existing structure. Generally, the flow of the shower water is vertically downward onto the stock, heretofore there having been no known attempt to locate the shower apparatus or orient the shower spray in such a manner as to cause the formation of uniform continuous closely spaced ridges useful in preventing reflocculation.
It is highly desirable to provide a shower apparatus which directs needle-like jets of water gently upon the stock as continuous streams at a position where the stock is carried over the forming board by the forming medium. It is also desirable to provide apparatus for deflocculating stock prior to the dewatering process. It is further desired to provide a uniform flow pattern in stock which has been dissipated upon a forming board and to cause such pattern to be in the form of continuous ridges which extend in the machine direction and are closely spaced in the cross machine direction. It is also desirable to provide a shower apparatus for deflocculating stock in the absence of any appreciable stock jump even in instances where the headbox discharge is non-uniform.